This invention relates to die bonding apparatus, and in particular to such apparatus incorporating a novel wafer table and using a novel means of transferring dies from a wafer table to a bond pad located on a leadframe.
In a die bonding apparatus a wafer table is provided for carrying a silicon wafer. The wafer is generally circular and is made up of a large number of integrated circuit devices known as dies which are arranged in a rectangular array. In a die bonding apparatus the dies are picked up from the wafer table by a pick and place device which transfers the dies one by one from the wafer to respective bond pads formed on a leadframe. The bond pad is provided with glue and the dies are pressed onto the bond pad to be located there. This process is shown schematically in FIG. 1 in which a die 1 is shown being transferred from a wafer 2 to leadframe 3 that is provided with a plurality of bond pads 4 each provided with glue 5.
Conventionally the dies are picked up from the wafer by the pick and place device at a fixed location, and the wafer is supported on a movable wafer table that allows the wafer to be positioned so that a die to be picked up by the pick and place device can be moved to the pick-up position from which the pick and place device is adapted to pick up the dies. As is shown in FIG. 2, the wafer table is adapted to move the wafer in mutually orthogonal X and Y directions, and for maximum speed the wafer table is adapted to move in both the X and Y directions simultaneously. In this way any die on the wafer can be moved easily to the pick-up position. Such conventional designs have proved perfectly acceptable in the past, but they have disadvantages with the larger size wafers that are becoming more common in the field.
Wafers are conventionally circular, and in the past conventional wafer sizes have been 6 inches and 8 inches in diameter. A 6 inch diameter wafer means that for any die on the wafer to be movable to the pick-up position, the wafer table must have a range of travel of approximately 6 inches in both the X and Y directions, while for an 8 inch diameter wafer the range of travel increases to 8 inches in both X and Y directions (in fact it is actually a little larger as space has to be allowed for other components such as the wafer expander). To provide sufficient space for this movement a wafer table for an 8 inch wafer must have an available space of about 600xc3x97600 mm2.
More recently larger wafer sizes are becoming available. Larger wafer sizes are preferable because they reduce the proportion of dies located at the edge of a wafer to the total number of dies on a wafer. Such edge dies are more likely to be faulty and so larger wafers will have a greater proportion of correctly functioning dies. Wafers of 12 inch diameter are now being produced.
Unfortunately such large wafers present serious difficulties with a conventional wafer table design that is adapted to position the dies at the pick-up position by movement along the XY axes. Such a wafer table would have to have a range of movement of the order of 12 inches in both the X and Y directions, and would require a total area available for movement of some 800xc3x97800 mm2 which unacceptably increases the total size and footprint of the die bonding apparatus.
According to the present invention there is provided die bonding apparatus comprising, a wafer table for supporting a wafer consisting of a plurality of dies, pick-up means for picking a die from a pick-up position and for transporting the die to a bonding location, and means for moving the wafer table whereby a selected die can be moved to the pick-up position, wherein said means for moving the wafer table comprises means for rotating the table and means for moving the table along one axis.
This method of moving a target die to the pick-up position minimises the amount of linear movement of the wafer table that is required and thus minimises the size and in particular the footprint of the apparatus. This is particularly advantageous with relatively large size wafers, but could equally be used with smaller wafer sizes.
The above described method will move a die to the pick-up position, and then at the pick-up position further fine adjustment of the position of the die may be necessary before it can be picked up. Therefore means may be provided for fine adjustment of the position of a die after it has been moved to the pick-up position. Such fine adjustment means may include a vision system.
While the basic movement of a target die to the pick-up position may be effected by rotary movement and by movement along one axis, it may be preferable to provide at least a small amount of movement along another axis normal to the first to facilitate this fine adjustment and therefore the fine adjustment means comprises means for moving along the one axis and along an axis at right angles to the first axis.
In a second embodiment of the invention, in addition to providing movement of the wafer table along a first axis and in a rotary sense, it may also be preferable to provide means for moving the wafer table along a second axis at right angles to said one axis. In such an embodiment a user may be able, at least for smaller size wafers, to choose between rotary movement and movement along one axis, or rectilinear movement along two axes, as the means for moving target dies to the pick-up position.
Using rotary movement to move a target die to the pick-up position may result in dies being in various orientations when they reach the pick-up position. Preferably therefore vision means is provided for sensing the orientation of a die to be picked-up relative to the first axis, and means are further provided for orienting the die to a desired orientation prior to being bonded. A further possibility is that, especially for dies with a high aspect ratio and which require multiple ejector pins, means may be provided for rotating the ejector pin assembly in accordance with the orientation of a die at the pick-up position.
Viewed from another broad aspect the present invention provides die bonding apparatus comprising, a wafer table for supporting a wafer consisting of a plurality of dies, pick-up means for picking a die from a pick-up position and for transporting the die to a bonding location, and means for moving the wafer table whereby a selected die can be moved to the pick-up position, said moving means being adapted to rotate said wafer table and to move said wafer table along two orthogonal axes, wherein said wafer table comprises interchangeable holding means for holding different sizes of wafers, and wherein said wafer table may be moved to move a die to the pick-up position either by rotation and movement along one of said axes or by movement along said two orthogonal axes at the choice of a user.
Such an embodiment provides a user with maximum flexibility as a single machine may be able to work on different size wafers but with the size minimising advantage of the present invention in respect of large wafers. For smaller wafers, users may be able to select their own choice of drive means for the wafer table.
Die bonding apparatus comprising, a wafer table for supporting a wafer consisting of a plurality of dies, pick-up means for picking a die from a pick-up position and for transporting the die to a bonding location, and means for moving the wafer table whereby a selected die can be moved to the pick-up position, said moving means being adapted to rotate said wafer table and to move said wafer table along two orthogonal axes, wherein said wafer table may be moved to move a die to the pick-up position either by rotation and movement along one of said axes or by movement along said two orthogonal axes at the choice of a user.
Viewed from another broad aspect the present invention provides a method of moving a target die in a wafer to a pick-up position in a die bonding process comprising the steps of rotating the wafer until the target die lies on a chosen axis, and linear movement of said wafer along said axis until said target die is at the pick-up poisition.